U.S. Pat. No. 4,566,345, issued Jan. 26, 1986 to the present inventors and disclosed a carrier for mounting a tool for reciprocating motion along a pair of spaced, parallel guide rails. The carrier itself is reciprocated bilaterally by a rotating lead screw extending lengthwise between and parallel to the guide rails. The lead screw engages an internally threaded nut on the carrier. The nut may be an anti-backlash nut and will be so illustrated hereinafter. With the lead screw located between the rails, there are bearing supports extending laterally from the carrier, each mounting a bearing which slides on a guide rail much in the manner of outrigger pontoons.
The rails serve a number of purposes. They create a low-friction guideway for the carrier to ride on but, more importantly, they create an anti-rotation mechanism for the carriage. If there were no guide rails and the carrier were mounted on the lead screw only, the carrier probably would not reciprocate. This is because the mass of the carrier would create high frictional resistance between its threaded nut and the lead screw such that the carrier would rotate with the lead screw rather than traversing it. Secondly, again, if there were no guide rails and if the lead screw were oriented in a horizontal direction, the flexure of the lead screw, created by the weight of the carrier and the tool that it mounts, could cause the carrier to bind on the lead screw and/or cause the tool that it mounts to disengage from its workpiece.
Yet another problem can exist. Tools mounted on the carrier are normally offset from the central axis of the lead screw. In addition, the tools are mounted on posts in a cantilevered position relative to the carrier. Normal drag of the tool relative to its workpiece creates a torque which is imparted through the tool post to the carrier and, hence, to the supports riding on the rails which carry the bearings. This can cause unnecessary drag, monkey-biting, or can conceivably result in the lead screw""s driving motor to stall out.
Another shortcoming in certain applications where space is limited is the size of the mechanism. Two guide rails spaced laterally of a lead screw creates a drive mechanism which is spread out, generally horizontally, and, in some machine operations, there is not sufficient space to do this. In addition, it is difficult to obtain initial alignment of the screw and two parallel rails.
It is to the solution of these problems that the present invention is directed.
The invention resides in a reinforced lead screw assembly which includes a threaded lead screw rotatable about a central axis by a reversible motor. Surrounding the lead screw is a rigid, hollow, elongated reinforcing rail or tube which extends lengthwise end to end of the screw. A two-part anti-backlash nut is movable along the reinforcing rail and is driven by the screw. There is a slot in the reinforcing rail which extends lengthwise of the central axis of the lead screw. Projections or tongues on both portions of the nut extend in a radial direction through the slot in the reinforcing rail and are engagable with the walls of the slot to prevent rotation of the anti-backlash nut relative to the rail. The tongues or projections mount threads which are engagable with the threads of the lead screw. Consequently, when the lead screw is rotated in either direction, the nut will translate lengthwise of the screw.
The projections on the nut portions have either partial threads engagable with the threads of the lead screw or mounted circular hubs which are internally threaded to engage the lead screw.
The reinforcing rail may include at least one flat extending lengthwise which is engagable with mating flats on the nut portion to supplement the anti-rotation function of the tongue and the slot in the rail to prevent the nut from rotating relative to the reinforcing rail. The outer surface of the reinforcing rail may be coated with a low-friction material such as PTFE, Nylon or the like. The anti-backlash nut also may be made of self-lubricating material such as acetyl and may include PTFE carbon fiber additive or other lubricating additives for low frictional engagement with the lead screw. It may be molded around or attached to a ball bushing for added stiffness and lubricity. The sleeve bearing located within the lead screw may be made of Nylon, Delrin or like plastic material.
A spacing mechanism such as an advancing collar positions and retains the nut halves or portions apart so that the flanks of their threads are in contact with the flanks of the threads of the lead screw to prevent backlash while the nut translates laterally along the reinforcing rail in both directions.